1. Field of the Invention
The present invention relates to a transflective liquid crystal display device, and more particularly to a transflective liquid crystal display device which arranges a transmissive region and a reflective region on a pixel part thereof.
2. Description of the Related Arts
As an active matrix type liquid crystal display device currently on a main stream, a reflective liquid crystal display device, a transmissive liquid crystal display device, and a semi transmissive and semi reflective liquid crystal display device (hereinafter, referred to as a transflective liquid crystal display device) which combines the reflective liquid crystal display device and the transmissive liquid crystal display device have been known. The transflective liquid crystal display device realizes advantages of the transmissive liquid crystal display device and the reflective liquid crystal display device using one liquid crystal display device by providing a transmissive region which allows the transmission of light from a back light and a reflective region which allows the reflection of an external light on a pixel part.
Japanese Patent Laid-Open No. 2003-315766 (patent document 1), as shown in FIG. 5, shows a cross-section of a pixel part of a so-called transflective liquid crystal display device, wherein a metal reflective film 41 (constituted by forming an Al film on a Mo film) is arranged as a pixel electrode of a reflective region, and an ITO layer 42 is arranged as a pixel electrode of a transmissive region.
Further, in patent document 1, as a countermeasure to cope with flickers which are conspicuously observed particularly when the transflective liquid crystal display device is driven with low frequency, as shown in FIG. 6, there is described the constitution which covers a surface of a reflective electrode 51 (Al) arranged in a reflective region with an amorphous transparent conductive film 52 made of InZnOx (oxide mainly containing indium oxide (In2O3) and zinc oxide (ZnO), work function: approximately 4.8 eV). Here, numeral 53 in FIG. 6 indicates an ITO film which constitutes a transmissive region. Usually, a work function of aluminum is 4.06 eV and a work function of ITO is 4.41 eV and hence, common potentials of the reflective region and the transmissive region are shifted from each other by approximately 0.4 V whereby flickers are generated. In the above-mentioned constitution, to suppress of the generation of the flickers, the ITO film is also arranged in the reflective region so as to make the work functions equal to each other thus eliminating the difference between the common potentials.
Aluminum described in patent document 1 is favorably used as metal for forming the reflective electrode. However, when aluminum is used as metal for forming the reflective electrode, the compatibility of the aluminum film with the transparent conductive film made of ITO or the like which is arranged over the aluminum film has to be taken into consideration. It is found that the constitution which arranges the ITO film on the aluminum film has a drawback that aluminum oxide is formed on an interface between the aluminum film and the ITO film due to electrical conductance therebetween and a charge is stored in the interface thus generating an image retention. Further, it is also found that such a constitution brings about not only the drawback on the generation of image retention but also a possibility that the electrical conductance is not achieved on the interface between the aluminum film and the ITO film and hence, the ITO film loses a function of an electrode whereby the difference in work functions is generated between the ITO film and an ITO film which is arranged in the transmissive region.
To cope with such drawbacks, it may be possible to arrange an ITO film having a small film thickness (for example, 30 μm or less). However, with such a thin ITO film, there exists a possibility of disconnection of the ITO film at a broken-step portion of a through hole which connects a transistor and a pixel electrode and hence, the countermeasure is not effective.
The present invention has been made to overcome such drawbacks, and it is an object of the present invention to provide a transflective liquid crystal display device which can reduce the generation of an image retention even when the transflective liquid crystal display device adopts the constitution in which an aluminum film is formed in a reflective region and a transparent conductive film made of ITO or the like is arranged over the aluminum film.